1. Field of the Invention
The invention is related to an apparatus for testing insulators and more particularly to an apparatus for detecting faulty insulators of the "suspension" type used on overhead power lines.
In the overhead transmission of electrical power, lines are supported from poles or towers by means of insulators made of dielectric material.
From these insulators, the ones commonly referred to as "end-of-line" type or "suspension" type are responsable for a high percentage of the power failures.
The combined effects of changes in temperature and humidity to which the insulators are constantly submitted, tend, over a period of time, to cause their deterioration. This deterioration being a function of both the micro-structure of each individual insulator and it's reaction to a changing environment, it is highly improbable that insulators at a specific location which are usually connected in strings of two or more all become defective at the same time.
In most cases, where two insulators are used, if one becomes defective, the remaining one can usually withstand the phase to ground voltage for a limited period of time if extreme conditions such as overvoltages, extreme contamination, etc, do not prevail.
An object of this invention is to provide means to detect faulty insulator so that it can be replaced before an other insulator of the same string becomes defective therefore creating a costly power failure.
Another object of this inventionis to provide means to detect faulty insulators on energized lines in such a manner that it becomes both time saving and safe for the operator.
2. Prior Art
Many methods and apparatus have been developed in the past to detect faulty insulators. One such method and apparatus is disclosed in U.S. Pat. No. 1,923,565. According to this patent, a forked probe is positioned across an insulator. A direct voltage, generated by a D.C. (direct current) generator (called a Megger), is applied to the insulator and the flow of current across a faulty insulator is detected and shown on a current indicator. The method and apparatus according to this patent have several disadvantages. The transmission line must be de-energized during the testing to avoid placing a high voltage on the test equipment and to ensure the safety of the personnel that carries out the testing. A separate source of power such as, for example, a battery must be used, in addition to using a Megger, to energize the primary induction coil of the transformer which forms part of the apparatus.
According to U.S. Pat. No. 1,943,391, insulation may be tested by impressing a potential sufficient to cause disturbance currents in the lack of homogeneities of the insulation property which is being measured. According to U.S. Pat. No. 2,281,470, apparatus for measuring high electrical resistance comprises a source of D.C., a rectifier for passing current flow from the D.C. source through the resistance to be measured and an instrument for measuring the current from the source to the resistance through the rectifier. The apparatus measures the resistance of a device and can not be used to detect faulty insulators on power transmission lines.
According to U.S. Pat. No. 2,239,598, grounded insulation is tested by applying an A.C. voltage and measuring current or capacitance. According to U.S. Pat. No. 2,923,879, insulators are tested by impressing an alternating voltage across an insulator and measuring the difference between two voltages which represents the resistive component of current through the insulator. According to U.S. Pat. No. 3,363,172 grounded insulators are tested by applying a test voltage and measuring the current flowing through the insulator; the testing means include a transformer and a grounded lead. Most of these disclosures involve grounding the insulator or applying an A.C. voltage.
According to Canadian Pat. No. 1,124,792, insulators can be tested on overhead power transmission lines while the power line remains fully energized. Though this tester utilizes a high voltage capacitor as an energy source in its measuring circuit, a few faulty insulators detection will drain its energy source, thus impairing further readings. Due to this unstable power source, the device will have to be recharged in situ which leads to a time-consuming operation and careful handling due to live probes.